Relative agronomic effectiveness of some African rock phosphate fertilizers in an ultisol of Okinawa, Japan

A greenhouse experiment was conducted to compare the agronomic effectiveness of four phosphate rock (PR) fertilizers from Togo (TPR), South Africa (SAPR) and Morocco (MPR) with single super phosphate (SSP) for corn (Zea mays L.) on an acidic P-deficient ultisol in Okinawa. The P rates used were 0, 100, 200, and 300 mg P kg^-1 soil and corn was grown for six weeks. The relative agronomic effectiveness (RAE) of each PR with respect to SSP was calculated. The dry matter yield (DMY) and P uptake followed the order SSP>MPR>TPR=SAPR=control. The RAE values ranged from 8 to 78% and were significantly correlated (r = 0.96**) with chemical reactivity of the PRs measured by extraction with ammonium citrate (pH 3). The DMY, P uptake and RAE values for MPR were statistically similar (P < 0.05) to SSP, suggesting that MPR can be used as a P source in Okinawa soils.